This invention relates to an orthopaedic surgical implant device or prosthetic component useful for long-term replacement of an articulating surface in a joint.
The articulating or bearing surfaces of bones in a joint are particularly susceptible to deterioration caused by injury or disease which leads to loss of movement and severe pain. It is generally known to surgically remove the damaged bone in a joint and attach a prosthetic component onto the resected bone surface wherein the component is anatomically designed to closely replicate the natural form and function of the articulating surface.
Consider by way of example, the surgical restoration of the knee joint often referred to as total knee arthroplasty. Total knee arthroplasty (TKA) is a surgical procedure wherein the damaged bone surfaces of the knee joint are removed and replaced with implant devices which facilitate accurate articulation at the restored joint so that a full range of motion including extension, flexion and some rotation in the joint is possible. To achieve normal movement in the joint, a separate component for the femur, tibia and patella corresponding in configuration to the natural articulating surface of the bones respectively is provided.
It is known to fabricate the aforementioned prosthetic components from high strength metallic materials such as titanium or chrome-cobalt which are known to exhibit biocompatibility. The metallic component is directly attached to the resected bone surface by a variety of methods. A plastic component is provided at the interface of the metallic surfaces to lower friction. For example, in Total Knee Arthroplasty the patellar and tibial devices are metal-backed plastic components. The metallic surface of the component is attached directly onto the bone surface and distributes load onto the related bone. The low-friction plastic surface provides the bearing or articulation surface in contact with the other surfaces in the joint. The femural component is a single metallic unit having a full range of motion in contact with the plastic bearing surfaces of the tibial and patellar components. Each implant device has screws, pegs, stems or other contrivance to aid in the fixation of the device to bone on a short-term or long-term basis. It is additionally known to secure the components to the bone with cement applied at the interface of the bone and prosthetic component. Newer art provides a means for firm attachment to the bone in the form of porous metallic or plastic coating on the attachment surface of the component which permits bone ingrowth.
Although the prior art implant devices have had success as functional replacements of removed tissue, it is known that over time the components tend to loosen. Component loosening may require further surgery to re-attach the component, or sometimes additional bone is removed and a new replacement component is attached. The tibial component, in particular, is known to experience component loosening over time which eventually leads to failure of the joint restoration.
Several tibial component designs have been developed to prevent loosening and component failure. More severe or harsh methods of attachment such as attaching the component onto the proximal end of the tibia with large spikes or posts have been used alone or in conjunction with cement. These designs are directed to preventing lateral or rotational movement of the component which may cause loosening over time. However, a large amount of bone must be removed to provide deep canals in the bone surface for the spikes or posts.
U.S. Pat. No. 4,808,185 issued to Penenberg, et al. discloses a tibial prosthesis developed to permanently fix the component to the bone. The distal surface of the implant device has domical contours which interengage and fit into corresponding domical recesses cut in the resected surface of the bone. Attachment is provided by screws placed through the component and into the proximal tibia at the domical recesses. A metallic coating as heretofore mentioned is also provided on the distal surface of the component to promote bone ingrowth. The component is designed to assure firm attachment to the bone by uniformly spreading forces through the tibia and preventing lateral movement of the component.
The prior art means have attempted to solve the problem of component failure by firmly attaching the component to bone, evenly distributing load across the surface of bone and preventing lateral or rotational movement of the component. However component loosening remains a problem, especially in the tibial replacements and further refinement is needed to improve implant longevity. In so doing the procedure could be offered to younger or more active patients who are presently not considered good candidates for TKA.
It is generally believed that one factor which contributes to component loosening and restoration failure is bone remodeling which prevents the bone from supporting the prosthetic component or patient activity level. The bone to which the prosthesis is attached is not a structurally homogeneous material, it has a wide range of mechanical properties. Bone has the ability to rearrange its entire structure in a pattern which will provide maximum strength while using a minimum of material. The resorption of old bone and generation of new bone occurs in a way which improves bone's ability to carry mechanical loads. Researchers have discovered that many of the design features incorporated in the tibial components including attachment means such as pegs and stems deleteriously alter the manner in which mechanical load is distributed. More importantly, the inventors have discovered that the relatively stiff implant devices of the prior art distribute load to the underlying bone differently than it is transferred by the natural bone. In the natural tibia, loads are transferred through the two condyles of the femur, to the medial and lateral regions on the tibial plateau. Analysis has shown that mechanical loads are applied to the natural intact proximal tibia in relatively localized areas. With current prosthetic devices, the stress in the proximal tibia is lower and more uniformly distributed. The relatively stiff component acts as a plate which distributes the load evenly across an elastic foundation. As the stiffness of the prosthesis is reduced, stress in the proximal tibia increases and as the stiffness of the component approaches that of bone, the stress in the proximal tibia approaches that of the intact natural tibia.
The bulk modulus of elasticity or the stiffness of the prior art metallic components is several orders of magnitude greater than that of the underlying bone. For example, cobalt-chrome and titanium which are typically used for orthopedic implants have a modulus of elasticity of 200,000 Newtons per millimeters squared (N/mm.sup.2) and 110,000 N/mm.sup.2 respectively. The densest cancellous bone in the proximal tibia is 350 N/mm.sup.2, nearly three orders of magnitude less stiff than either of the metals. The mechanical stress and strain of the applied load at the flat interface of the component and proximal tibia causes abnormal bone growth and detrimentally affects the load carrying capability of the surrounding bone in the long term implant situation. Additionally, shear and tensile stress is experienced at the flat plateau interface of the proximal tibia which prevents biologic attachment of the component through bone ingrowth.
Therefore, it is a primary object of the present invention to provide a orthopaedic surgical implant device for replacing the articulating surface of a joint that distributes load and stresses onto the underlying bone as it is normally distributed in a natural joint.
It is also an object of the present invention to provide an orthopedic surgical implant device for replacing articulating surfaces of a joint that fosters normal bone growth in the proximal tibia so the bone can support the device.
It is still another object of the present invention to provide an orthopaedic surgical implant device that will remain attached to the bone for a relatively long time thereby improving implant longevity and permitting increased physical activity.
It is still a further object of the present invention to provide an orthopaedic surgical implant device having a stiffness factor more closely related to the stiffness of bone so that the device does not adversely affect the bone to which the device is attached.
It is still another object of the present invention to provide an orthopaedic surgical implant device having a curvature which minimizes bending stress in the device and prevents mechanical failure of the device.
It is another object of the present invention to provide an orthopaedic surgical implant device that produces a more physiological load application in the underlying bone by optimally reducing the cross-sectional mechanical properties of the device, particularly cross-sectional thickness and resistance to bending.
These and other objects of the present invention are achieved by an orthopaedic implant device designed to replace an articulating surface in a joint, wherein the device has a low stiffness factor which is capable of applying normal physiological load onto the underlying bone. The component is made of a material having a bulk modulus of elasticity nearer in magnitude to that of the underlying bone than has been that of previous components. Preferably the component will be manufactured from materials two orders of magnitude less stiff than high-strength metallic materials such as cobalt-chrome or titanium. To meet this objective, the component is preferably made from an advanced material such as polyetheretherketone, polysulfone or ultra-high molecular weight polyethylene (UHMPE). A low-stiffness factor is also achieved by a relatively thin component. The device is provided with curvature to minimize bending stress in the relatively thin component so that the component has sufficient mechanical strength to prevent stress induced failure. The curved configuration additionally provides a component having relatively low shear stress at the interface of the device and the bone.
A tibial implant device made in accordance with the present invention is a bicondylar concave-shaped unitary component made of an advanced material and having a related cross-sectional thickness that defines a normalized stiffness of less than 50,000 Newtons per millimeter which is more commonly expressed as 50,000 N/mm. The upper surface of the component is generally concave-shaped and designed in conjunction with the femur or femural implant to provide normal knee flexion and extension. The distal surface of the component is generally parallel to the upper surface, providing domical configurations on the distal surface in the medial and lateral regions of the component. Corresponding concave recesses in the proximal tibial bone surface are provided which receive the domical configurations on the distal surface of the implant when attached to the bone. Pegs or screws may be provided at the mid-sagittal region of the component to provide short term attachment. A porous thermoplastic coating on the distal surface of the component is preferred to promote bone ingrowth for long-term attachment.